Ventilation of the abyss in the Atlantic sector of the Southern Ocean

The Atlantic sector of the Southern Ocean is the world's main production site of Antarctic Bottom Water, a water mass that is ventilated at the ocean surface before sinking and entraining older water masses - ultimately replenishing the deepest layers of the global overturning circulation. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in the Atlantic sector of the Southern Ocean have led to a strikingly broad range of results, with water transport-based calculations (8.4-9.7 Sv) yielding much larger rates than tracer-based estimates (3.7-4.9 Sv). Here, we reconcile these conflicting views of Antarctic Bottom Water production by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly identified and quantified. We show that the layer of Antarctic Bottom Water denser than 28.35 kg m^-3 γⁿ is exported northward to the Atlantic Ocean at a rate of 8.7 ± 0.9 Sv, composed of (i) 4.8 ± 0.4 Sv of well-ventilated Dense Shelf Water and (ii) 3.9 ± 0.5 Sv of old, mid-depth waters entrained into cascading plumes of Dense Shelf Water (3.4 ± 0.8 Sv) is generated in the continental shelves of the Weddell Sea. Entrainment of old waters at mid-depth into this locally formed Dense Shelf Water plume occurs at a ratio of 2.15 ± 0.5. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector of the Southern Ocean, and define a baseline, process-discerning target for its realistic representation in climate-scale ocean models.